Physicists find evidence that the universe isn’t perfectly uniform, potentially unraveling 100-year-old model of cosmology
Astronomers have developed a new way to test one of the central hypotheses of modern cosmology: that the universe behaves uniformly at the largest scales. By applying the method to real observational data, the researchers found tentative signs that this hypothesis may not be fully verified, which could indicate new physics beyond the realm of physics. standard cosmological model.
The work combines observations of distant exploding stars and studies of large-scale galaxies to determine whether the universe actually follows a nearly 100-year-old mathematical framework known as Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology. The analyzes revealed small but intriguing deviations from the standard model predictions.
“We found a surprising violation of an FLRW curvature consistency test, hinting at new physics beyond the Standard Model,” study co-author Asta Heinesenphysicist at the Niels Bohr Institute in Copenhagen and Queen Mary University of London, told Live Science by email, referring to the hypothesis that the curvature of space is the same everywhere. “This could potentially be due to various effects, but further research is needed to address the cause of the FLRW violation that we observe empirically.”
The results were presented in a series of three papers which introduce new diagnostic tests for cosmology and apply them to existing observational datasets. The articles, available on the arXiv preprint server, have not yet been peer-reviewed.
Testing the foundations of cosmology
Modern cosmology is based on the assumption that, when viewed at large enough scales, the universe is homogeneous and isotropic, meaning that matter is distributed evenly and that the cosmos is roughly the same in all directions. This idea underpins FLRW cosmology, which forms the basis of the Standard Model of cosmology, known as lambda cold dark matter.
But the real universe contains a tangle cosmic web of galaxies, clusters of galaxies and enormous empty regions called voids. According to Heinesen, this complexity means that the FLRW description does not always apply perfectly.
“FLRW cosmology assumes a spacetime with maximally symmetric spaces,” Heinesen said. “It is necessary to go beyond FLRW spacetimes when cosmological structures are present such as galaxy clusters and voids of empty space.”
The researchers focused on two possible effects that could distort the apparent geometry of the universe. One of these is the Dyer-Roeder effect, which occurs because light from distant objects often travels primarily through empty regions of space rather than through material-rich environments. This could lead physicists to ignore much of the universe’s matter density, “which would make the universe appear emptier than it actually is,” Heinesen explained.
The second possibility involves an effect called a cosmological reaction. In this scenario, the growth of large-scale cosmic structures changes the average expansion of space itself.
DESI’s 3-year map of the universe shows the distribution of matter in space-time. New studies based on DESI and other survey data suggest that our standard model of cosmology may need an update.
(Image credit: DESI Collaboration/DOE/KPNO/NOIRLab/NSF/AURA/R. Proctor)
A new way to probe cosmic geometry
To investigate these possibilities, the researchers performed mathematical consistency tests designed to check whether the observational data obeys the rules expected in an FLRW universe. In particular, they used variations of the Clarkson-Bassett-Lu test, a method that compares measurements of cosmic distances and expansion rates.
The team developed a more general framework that works even when the universe doesn’t follow the FLRW assumptions perfectly.
They also introduced machine learning techniques known as symbolic regression to reconstruct cosmic expansion histories directly from observational data. Instead of assuming a predefined cosmological model, the method searches for mathematical expressions that best fit the data.
Using observations from the Pantheon+ supernova catalog, as well as measurements from the Dark Energy Spectroscopic Instrument (DESI) — a major international project that mapping millions of galaxies across the universe — researchers have reconstructed how quickly the cosmos has expanded over time. They also used data from studies of baryonic acoustic oscillations, which track ancient patterns of galaxy distribution left by sound waves that passed through the hot plasma of the early universe.
The analyzes revealed minor but potentially important deviations from the predictions of standard FLRW cosmology. Depending on the data set and analysis method, the discrepancy reached a statistical significance of approximately 2 to 4 sigma. In physics, sigma measures the probability that an outcome will occur purely by chance; a 5-sigma result is usually required before scientists claim a discovery, so new findings remain tentative. Still, the results suggest that something unexpected could affect the geometry or expansion of the universe.
“The main finding is that you can directly measure Dyer-Roeder and backlash effects from available cosmological data, and clearly distinguish these effects from other alterations to the standard cosmological model, such as the evolution of dark energy and theories of altered gravity,” Heinesen said. “This was not possible before in such a direct way, and this is what I think is a major step forward in our work.”
Challenges and future directions
Researchers cautioned that the evidence remains preliminary. Current cosmological data are still relatively rare, particularly for measurements of the expansion rate of the universe at different times. Symbolic regression methods also introduce uncertainties that require further study.
In these articles, the authors emphasize that improving observations from future investigations will be essential in determining whether apparent FLRW violations are genuine.
“If these indicated deviations from FLRW geometry are real, it would mean that most of the cosmological solutions considered to resolve cosmological tensions – evolving or interacting dark energynew types of matter or energy, altered gravity, and related ideas within the FLRW framework—are excluded,” the researchers wrote.
The next step will be to apply the new theoretical framework to larger and more precise data sets. “This is about applying our theoretical results to the data to test the standard model and produce constraints on the Dyer-Roeder and feedback effects,” Heinesen said.
Since the method can already be used with existing astronomical observations, researchers may soon obtain more precise answers about whether the universe actually follows the simple large-scale picture assumed by standard cosmology or whether hidden complexities are reshaping our understanding of cosmic evolution.
